1. Field of the Invention
The invention is generally related to frequency synthesizers, and more particularly, is related to systems and methods for controlling the frequency and amplitude of an oscillator.
2. Related Art
Frequency synthesizers are regularly employed in communication transceivers used in numerous types of communication systems and communication technologies, such as, radio frequency communication systems. Typically, such frequency synthesizers use a phase-locked loop to precisely control the frequency of the output signal of a voltage-controlled oscillator (VCO). The frequency synthesizer typically consists of a VCO, a low pass filter, and a phase-locked loop. The VCO generates an output signal having a particular frequency. A phase detector in the phase-locked loop receives the output signal and compares the output signal to a reference frequency. Based on the comparison of the output signal to the reference frequency, the phase-locked loop generates a control signal that is provided to the low pass filter and then to the VCO. The control signal is typically received by a variable capacitor, referred to as varactor, in the VCO. The control signal tunes the variable capacitor, thereby changing the frequency of the output signal of the VCO.
Generally, such frequency synthesizers are effective where the range of the frequency of the output signal is limited. However, in situations where the frequency of the output signal of the VCO needs to be varied over a broad frequency range, such frequency synthesizers are very problematic. For example, where frequency synthesizers are used in wireless broadband and wire-line broadband communication systems, or in multi-band devices and multi-mode devices, the limited tuning range of the varactors prevents effective tuning of the VCO and, therefore, effective channel selection. The limited tuning range of the varactors may also be problematic in situations where large fabrication process variations, temperature variations, or other variations require the output of the controllable oscillator to be varied over a broad frequency range. Additionally, varactors with a wide-tuning range are also problematic due to the fact that they are extremely sensitive to noise and interference on the control lines from the phase-locked loop. Furthermore, the non-linear characteristics of wide-tuning varactors are problematic in the design of the loop filter in the frequency synthesizer. Thus, there is a need in the industry to address these deficiencies and inadequacies.